The demands of the environment exert extraordinary influence on the practice of medicine at sea. Even at sea, medicine is still medicine: history and examination are necessary for diagnosis, use of diagnostic adjuncts must be weighed against resources, and treatment is based on universal principles of surgery and medication. And yet, being at sea is different. The stress of close living quarters, isolation, and a hazardous environment are unequaled short of space travel. The prolonged absence from home, community, and normal environment creates profound emotional stress. It also precludes normal exposure to minor infections, rendering an entire crew not only immunologically isolated but also immunologically naïve, compared with shore populations. The cramped living and working spaces create complicated and unnatural challenges for hygiene, nutrition, and the control of contagion. It is also a unique industrial environment, which carries yet further medical concerns.